Pipe strings being assembled in earth bore holes are supported at or near the upper end by spiders that rest on the derrick floor, or by elevators that are suspended from the rig traveling block. The pipe is added to the string, usually in lengths up to three sections if going into the hole, and usually by single sections while drilling. Removal of the pipe string from the hole proceeds in reversed steps, usually in lengths of two or three sections. A section is an uninterrupted length of about thirty feet, and is often called a joint.
Pipe string loads are most safely supported by engaging plane or tapered surfaces on the pipe string. Such surfaces, however, are not always in a suitable position relative to the spiders or elevators and the pipe string has to be supported by gripping the outer cylindrical surface of the pipe.
A pipe load support assembly, known as the “fail safe system” comprises a spider with a generally central opening to accept vertically situated pipe. The opening is defined by a slip bowl surface that is conical, opening upwardly. A plurality of slips is distributed peripherally about the slip bowl surface. Slip handling gear moves the slips vertically along the slip bowl surface. The vertical movement forces a proportionately less radial movement. The slips, each, have gripping surfaces situated to contact the outer surface of the pipe in the opening. The gripping surfaces are usually on dies that attach to the slips. A plurality of dies, usually three or more, are vertically distributed on each slip.
The slips are effectively wedges that are supported in conical bores of slip bowls in the spider body. The vertical force on the slips results in a proportionally greater radial force thrusting each slip toward the pipe to be gripped. The surface of the pipe is gripped by teeth, or the equivalent, supported on the slip. The teeth wear and occasionally have to be replaced. To enable tooth replacement on the slips, the slips usually have surfaces to accept, capture, and support dies with teeth adapted to the requirement of the pipe load and surface to be gripped.
Spiders often have “fall back” openings near the conical surfaces of the slip bowl to allow slips to move radially outward after a small amount of lifting to release the grip on pipe. That fall back allows enlargements on the pipe string to pass through the spider opening. Such enlargements may include couplings. The “fall back” arrangement increases the size of enlargements that can pass through the spider but does not increase the range of diameters that the spider can safely grip without changes in spider configuration.
On modern pipe strings there are many enlargements, other than couplings, that occur in such positions that they need to be gripped by the spider to enable the rig pipe tongs to function. Otherwise stated, pipe strings now consist of more than just pipe couplings. There is now a need to use the spider for an extended gripping range in terms of diameter.
Gripping range, in terms of diameter, is influenced by the manner of gripping such that pipe surfaces will not be damaged when very large pipe string loads must be supported. To maximize the load supporting ability of pipe being gripped, the points of application of radial load needs to be equally distributed about the periphery of the pipe being gripped. Ideally, the optimal distribution of radial loads should not change due to changes in pipe diameter.
Pipe strings supported by gripping dies often receive substantial torque transmitted from the spider to the pipe. Often, the torque is collateral with other rig floor activities. Gripping dies that have teeth on a cylindrical surface that approximates the pipe outer cylindrical surface, when torque is being transmitted to the pipe, tilt somewhat as a result of machine slack and strain. When the dies tilt, one edge tends to gouge into the pipe. The resulting load concentration tends to distort the pipe with unplanned consequent pipe surface damage. Vee shaped dies do not contact pipe with an edge and load concentrations are distributed over more die surface. The resulting two vertical lines of die and pipe surface contact has a stabilizing effect. There is still some tilt from slack and strain but with less unexpected tendency to distort or damage pipe being gripped.